fix(compiler): add an empty content for source file of non mapped code.

[tbosch]

Before this when using ngc, tools tried to load ng://…<component>.ts
if …<component>.ts was the source file of a template.

Please check if the PR fulfills these requirements

• The commit message follows our guidelines: https://github.com/angular/angular/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (check one with "x")

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Other... Please describe:

What is the current behavior? (You can also link to an open issue here)

What is the new behavior?

Does this PR introduce a breaking change? (check one with "x")

[ ] Yes
[ ] No

If this PR contains a breaking change, please describe the impact and migration path for existing applications: ...

Other information:

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